Hydrometallation

Addition of hydride bonds of main group metals such as B—H, Mg—H, Al—H, Si—H and Sn—H to alkenes and alkynes to give 513 and 514 is called hydrometallation and is an important synthetic route to compounds of the main group metals. Further transformation of the addition product of alkenes 513 and alkynes 514 to 515,516 and 517 is possible. Addition of B—H, Mg—H, Al—H and Sn—H bonds proceeds without catalysis, but their hydrometallations are accelerated or proceed with higher stereoselectivity in the presence of transition metal catalysts. Hydrometallation with some hydrides proceeds only in the presence of transition metal catalysts. Hydrometallation starts by the oxidative addition of metal hydride to the transition metal to generate transition metal hydrides 510. Subsequent insertion of alkene or alkyne to the M—H bonds gives 511 or 512. The final step is reductive elimination. Only catalysed hydrometallations are treated in this section. 

These reactions can be explained by the following mechanism. At first, isopropyltitanocene (521) is formed by transmetallation and its -elimination generates Cp2Ti-H and propylene. Insertion of the alkene to Ti-H affords the alkyltitanium 522. Then the alkyl Grignard reagent 523 is formed by transmetallation 

Hydrozincation of terminal alkene with Et2Zn, catalysed by Ni(acac)2, yields the dialkylzinc 525, which reacts with the nucleophile 526 via transmetallation involving Cu salt to give 527 [199]. In this reaction, the Ni hydride is generated by / -elimination of EtNi(acac), and the hydronickellation product 524 is formed by alkene insertion to the hydride H—Ni(acac). Then transmetallation of 524 with Et2Zn affords 525. 

Hydroboration of alkenes with BH3 and R2BH proceeds at room temperature without a catalyst. As an exception, the reaction of the less reactive catecholborane (CBH, 529) proceeds at 70-100 °C. It was found that the slow reaction of CBH is accelerated by a Rh catalyst, and the catalyzed and uncatalyzed hydroborations give different products [200]. By the uncatalyzed reaction, the ketone in the enone 530 is attacked to form 531. On the other hand, the alkene is hydroborated to give 532 by the catalysis of RhCl(Ph3P)3 [201]. Different regio- and stereoselectivities are observed in the Rh-catalysed hydroboration of cyclohexenol (533) with catecholborane and uncatalysed reactions with 9-BBN-H [202], 

Dicarbonyltitanocene [Cp2Ti(CO)2] is also active for the hydroboration of alkynes at room temperature. Dimethyltitanocene [Cp2TiMe2] catalyses the hydroboration of alkene with CBH at room temperature [203], 

Mercury(II) salts317 and Tl(OAc)3318 may be used as catalyst to add aliphatic and aromatic amines to alkynes to yield imines or enamines. Selective addition to the carbon-carbon triple bond in conjugated enynes was achieved by this reaction 319 

Johannsen and co-workers found that the phosphoramidite of axially chiral BINOL (IH-9) is an effective chiral ligand, and they obtained 32 with 99 % ee in 87 % yield from styrene. Also they prepared 1-trichlorosilyl-l-phenylpropane (35) with 98 % ee in 91 % yield by the hydrosilylation of /S-methylstyrene (34) with this ligand, and chiral 1-phenyl-1-propanol was prepared from 35 [21]. Furthermore, they claimed that arylmonophosphinoferrocene was an efficient ligand. In particular, the p-MeO-Ph-MOPF (VIII-8) they synthesized was the most effective ligand for asymmetric hydrosilylation of styrene, and ultrafast asymmetric hydrosilylation occurred with TOF exceeding 180000 h [22]. 

As an extension of hydrosilylation of alkenes, cyclization-hydrosilylation of 1,6-dienes occurs by the reaction with HSiRs [23]. The cationic complex 37, generated in situ from (phen)Pd(Me)Cl and NaBAr4, is an active catalyst, catalyzing the reaction of dimethyl diallylmalonate (36) with HSiCl3 to give the disubstituted cyclopentane 38 with 98% trans selectivity in 92% yield [24]. A mechanism different from that of usual hydrosilylation, which postulates formation of H-Pd-SiR3 and hydropalladation of alkene, was proposed by Widenhoefer 

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Hydrometallation is catalyzed by Pd. Hydroboration of l-buten-2-methyl-3-yne (197) with catecholborane (198) gives the 1,4-adduct 199 with 84% selectivity. The ratio of Pd to phosphine (1 1.5) is important[l 10]. The vinyl sulfide 201 is prepared by a one-pot reaction of the thioalkyne 200 via a Pd-catalyzed hydroborution-coupling sequence using dppf as a ligand[l 11]. 

Metal-catalyzed hydrometallations in formation of heterocycles 98PAC1059. 

Hydrometallation and carbometallation of alkynylsilanes proceeds regio-and stereospecifically, the metal becoming attached to the silicon-bearing carbon atom in what is normally a co-addition process (hydrostannylation, however, shows the opposite regioselectivity). Electrophilic cleavage, with retention, of the carbon-metal bond then leads to vinylsilanes of various types. 

A variety of routes are available for the preparation of allylsilanes (/) with the simplest and most direct being the silylation of allyl-metal species. Other routes exemplified in this chapter include Wittig methodology, the use of silyl anions/anionoids in allylic substitution, and hydrometallation of... 

The isomerization of internal alkenes to terminal ones before hydrometalation or the isomerization during hydrometalation results in the formation of terminal prod-... 

The insolubility of 1 has the major inconvenience that it prevents studying in detail the mechanistic factors which would operate during this hydrometalation process, as in general only the organic substrate and final zirconocene product are observed by NM R spectroscopy. 

In marked contrast to the other hetero atom multiple bond functions cited in this section the >C=N- imine bond was not found to undergo hydrometalation. Imines with neighboring C-H bonds as in PhCH=NMe do react with 1 via imine/enamine tautomerism, but not by hydrozirconation [197]. 

Some hydrometalation reactions have been shown to be catalyzed by zirconocene. For instance, CpiZrCf-catalyzed hydroaluminations of alkenes [238] and alkynes [239] with BU3AI have been observed (Scheme 8-34). With alkyl-substituted internal alkynes the process is complicated by double bond migration, and with terminal alkynes double hydrometalation is observed. The reaction with "PrjAl and Cp2ZrCl2 gives simultaneously hydrometalation and C-H activation. Cp2ZrCl2/ BuIi-cat-alyzed hydrosilation of acyclic alkenes [64, 240] was also reported to involve hydrogen transfer via hydrozirconation. 

Since the initial reports of Schwartz s reagent ([Cp2Zr(H)Cl]n, 1) over 30 years ago there has been explosive growth of zirconocene chemistry. Hydrozirconation is an efficient method of hydrometalation because of (i) the mild nature of the conditions involved, (ii) the excellent regio- and stereo-chemical control of hydrozirconation, (iii) the one-pot nature of the procedure, and (iv) the price of Zr, which is one of the... 

Deliberate production of (vinyl)polystyrene from (toluenesul-foxyethyl)polystyrene or (haloethyl)polystyrenes was best accomplished by quaternization with N,N-dimethylaminoethanol, followed by treatment with base beta-deprotonation is encouraged in the cyclic zwitterionic intermediate. Reaction was faster and cleaner than with other reagents recommended (64, 76, 77) for eliminations, such as alkoxide, diazabicycloundecene or quaternary ammonium hydroxide this new and efficient procedure may find application elsewhere. Hydrometallation or other additions to polymer-bound olefin may prove useful steps in future syntheses by polymer modification. 

In addition to the metallacycle-based mechanism depicted above, stepwise insertion pathways such as alkyne hydrometallation followed by aldehyde... 

A variety of alternate methods for the reductive coupling of aldehydes and alkynes have been developed. A number of important hydrometallative strategies have been developed, although most of these methods require the stoichiometric formation of a vinyl metal species or metallacycle. A very attractive hydrogenative coupling method has recently been developed, and its scope is largely complementary to the nickel-catalyzed methods. A very brief overview of these methods is provided below. 

Diastereoselective reductive coupling of MVK and p-nitrobenzaldehyde performed under an atmosphere of elemental deuterium provides an aldol adduct incorporating a single deuterium atom at the former enone f>-pos-ition [69]. Deuterium incorporation at the a-carbon is not observed, excluding Morita-Baylis-Hillman pathways en route to product. Incorporation of a single deuterium atom suggests irreversible enone hydrometallation (Scheme 5). 

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